1. The Field of the Invention
The present invention is related to methods for reducing nitrogen oxide ("NO.sub.x ") emissions from pollution sources, such as combustion systems. More particularly, the present invention relates to the noncatalytic, selective reduction of NO.sub.x by --NH and --CN containing compounds to achieve very low levels of NO.sub.x emissions.
2. Related Applications
This application is a continuation-in-part application of copending application Ser. No. 06/039,324, filed Apr. 16, 1987, entitled "METHODS OF REDUCING NO.sub.x AND SO.sub.x EMISSIONS FROM COMBUSTION SYSTEMS" invented by Michael P. Heap, Shih L. Chen, James M. McCarthy, and David W. Pershing. That application is incorporated herein by this reference.
3. The Background of the Invention
One of the major problems in industrialized society is the production of air pollution from numerous sources. Air pollution can take various forms. Some of the different types of air pollutants include particulate emissions such as coal ash, partially burned coal particles, and the like, sulfur compounds such as SO.sub.2 and SO.sub.3 (sometimes collectively referred to as "SO.sub.x "), ozone, carbon oxide emissions, volatile hydrocarbon emissions, and emissions of nitrogen oxides (commonly referred to collectively as "NO.sub.x "). Pollution sources include automobiles, industrial plants, small commercial establishments, such as dry cleaners and service stations, and even nature itself.
Combustion effluents and waste products from particular types of sources have proven to be major contributors to damaging air pollution when the effluents are discharged into the atmosphere. Unless these waste products are treated before their release into the atmosphere, serious smog and air pollution problems are encountered.
It will be appreciated that high concentrations of air pollutants have serious deleterious impacts on the health and general welfare of society. Air pollution is known to aggravate certain medical conditions (such as heart and lung problems) and is known to cause problems in the environment, ranging from corrosion to acid rain.
One of the most common components found in polluted air is nitrogen dioxide ("NO.sub.2 ") which is known to be toxic. Nitrogen dioxide, which is brown in color, undergoes a series of reactions, known generally as "photochemical smog formation," in the presence of sunlight and airborne hydrocarbons. These reactions result in a marked decline in overall air quality.
While NO.sub.2 is produced from a wide variety of pollution sources, its primary source is from nitric oxide ("NO"") released into the air. NO is commonly formed during combustion processes, including internal combustion engines in automobiles, hydrocarbon fuel power plants, process furnaces, incinerators, coal fired utility boilers, glass furnaces, cement kilns, oil field steam generators, gas turbines, and other similar installations.
There are two primary mechanisms for the formation of nitrogen oxides in the combustion processes. Within the high temperature portions of flame, atmospheric oxygen can react with molecular nitrogen ("N.sub.2 ") to form NO by the high temperature "thermal fixation" mechanism.
In addition, fuels which contain large amounts of nitrogen chemically bound within the fuel structure may produce significant NO.sub.x emissions as a result of the oxidation of the fuel nitrogen during the burning process. This source of NO.sub.x emission (often termed "fuel NO.sub.x ) is the predominant source of NO.sub.x with the combustion of coal, heavy oils, biological and agricultural residues, and some municipal, industrial, and agricultural wastes.
Since NO is the only oxide of nitrogen which is stable at the high temperatures encountered in these types of combustion processes, NO is the predominant nitrogen emission product. At normal atmospheric temperatures, however, the equilibrium between NO and NO.sub.2 favors NO.sub.2. Hence, NO formed by combustion is generally discharged into the atmosphere as NO, and only subsequently converted to NO.sub.2. In order to control NO.sub.2 emissions, therefore, it is necessary to eliminate NO before it enters the atmosphere.
There have been considerable efforts in the art to find effective ways to remove oxides of nitrogen from waste gases so that these waste gases may be discharged to the atmosphere without harm to the environment.
Because the "thermal fixation" of atmospheric nitrogen is exclusively a high temperature phenomenon, occurring above 2800.degree. F., it has been possible to achieve significant reductions in NO.sub.x emissions from the combustion of nitrogen-free fuels (such as natural gas or gasoline) by reducing the overall temperature in the combustion zone. This is accomplished using techniques such as exhaust gas recirculation in automobiles or flue gas recirculation in utility boilers.
Fuel NO.sub.x formation is most easily controlled by limiting the amount of oxygen present during the period in which the nitrogen species are being evolved from the fuel matrix. Techniques such as a staged combustion, overfire air addition, and "burners out of service" all use this concept to limit fuel and nitrogen oxidation.
More recently, it has been recognized that limited amounts of hydrocarbon fuels, particularly those which do not contain fuel nitrogen, can be used to effectively incinerate NO formed in the main combustion zone by creating a fuel rich (oxygen deficient) environment downstream of the primary combustion zone. This technique is generically termed "reburning," and like the other combustion modification techniques, is capable of producing overall NO.sub.x reductions in excess of 50% under optimized conditions.
Unfortunately, at the present time, none of the combustion modification techniques are capable of producing very high levels of NO.sub.x control in the range of approximately 80% to 90%. To achieve extremely low NO.sub.x emission levels, it is necessary to utilize some type of downstream, effluent gas cleanup system.
It has been found in the art that removal of NO.sub.2 from a combustion effluent stream is relatively easy since it reacts with water and air to form nitric acid. NO.sub.2, therefore, is commonly removed by aqueous scrubbing. If a base, such as ammonia, is added to the scrub water, the nitrogen scrubbing process is facilitated and ammonium nitrate is produced. If limited amounts of NO are present along with the NO.sub.2, the NO may be coscrubbed, thereby yielding ammonium nitrate.
Most chemical scrubbing techniques are subject to the limitation that they are only effective for mixture of nitrogen oxides which are predominantly NO.sub.2, rather than predominantly NO. This presents a problem because NO is the predominant species at the high temperature generally encountered in flue gases. As a result, various processes have been developed in the art for oxidizing NO to NO.sub.2 so that the relatively inexpensive and convenient scrubbing processes may take place.
Several processes known in the prior art involve contacting the gaseous flow which includes NO, with various organic compounds (such as aldehydes, alcohols, ketones, organic acids, and the like) in the presence of oxygen. By such processes, the NO is oxidized to NO.sub.2 which can then be removed by scrubbing as described above. None of these processes, however, are capable of efficiently producing very low levels of NO.sub.x emissions.
An alternative approach for removing NO from flue gases and other streams of pollutants is to reduce NO to nitrogen and water, which may then be discharged to the atmosphere. Reduction of NO.sub.x may be accomplished with or without catalytic assistance. Practically, the noncatalytic processes are preferable because they are not subject to the usual disadvantages of employing catalysts. Some of these additional disadvantages include higher expense associated with the catalyst, the potential of catalyst plugging, the expense and difficulty of contacting the combustion effluents with the catalyst, and the danger that the catalyst will disintegrate and be emitted into the atmosphere.
Alternatively, NO.sub.x reduction processes often teach the removal of NO.sub.x from flue gases by reduction of the NO by the addition of ammonia, urea, or ammonia precursors, alone or in combination with some other combustional material, while the waste gas is at a relatively high temperature (generally from about 700.degree. C. to about 1200.degree. C.).
An example of such an NO reduction process is described in U.S. Pat. No. 3,900,554 to Lyon, issued Aug. 19, 1975, entitled "Method for the Reduction of the Concentration of NO in Combustion Effluents Using Ammonia." The process disclosed in that patent teaches the reduction of NO to N.sub.2 by injecting ammonia under excess oxygen conditions into the combustion effluent stream at a temperature from about 870.degree. C. to about 1100.degree. C. If the ammonia is injected along with a second reducing agent, such as hydrogen, NO will be rejected at temperatures as low as 700.degree. C.
A corresponding NO reduction process is described in U.S. Pat. No. 4,335,084 to Brogan, issued June 15, 1982, entitled "Method for Reducing NO.sub.x Emissions from Combustion Processes." The process disclosed in that patent is somewhat analogous to that disclosed by Lyon in that NO is reduced by a selective, noncatalytic, reaction with ammonia; however, according to the Brogan process, the reaction occurs under fuel-rich conditions and the reaction temperature window is considerably higher (1900.degree.-3000.degree. F.).
Recent data incidate that similar results can be achieved under both fuel-rich and fuel-lean conditions with urea injection. In U.S. Pat. No. 4,208,386 to Arand et al., issued June 17, 1980, entitled "Urea Reduction of NO.sub.x and Combustion of Effluents," a method for selectively reducing NO.sub.x in combustion effluents containing at least 0.1 volume percent oxygen at temperatures in excess of 1300.degree. F. is described. As with the Lyon invention for ammonia, the optimum temperature window for urea injection under excess air conditions is relatively low (1300.degree.-2000.degree. F.).
In a subsequent U.S. Pat. No. 4,325,924, again to Arand et al., issued Apr. 20, 1982, entitled "Urea Reduction of NO.sub.x in Fuel Rich Combustion Effluents," the authors disclose the existence of a high-temperature window (1900.degree.-3000.degree. F.) where urea can also be used to selectively reduce NO.sub.x emissions under fuel-rich conditions.
The above-referenced material demonstrates that while NO.sub.x emissions can be selectively reduced by ammonia, ammonia producing compounds and urea, the optimum temperature appears to depend primarily on whether selective reduction reactions are being conducted under fuel-rich (high temperatures required: 1900.degree.-3000.degree. F.) or fuel-lean conditions (moderate temperatures: 1300.degree.-2000.degree. F.). However, none of the above references disclose a process for achieving very low concentrations of NO.sub.x emissions.
Another group of pollutants which are of major importance are the sulfur oxides (generally collectively designated "SO.sub.x "). Sulfur oxides are primarily emitted in the form of sulfur dioxide ("SO.sub.2 "), with small amounts of accompanying sulfur trioxide ("SO.sub.3 "). Since there is no harmless gas phase sulfur species analogous to N.sub.2, combustion modification has not been useful for controlling SO.sub.x emissions. Exhaust gas cleanup systems, however, including both wet scrubbing and spray drying techniques, are well known and effective.
High temperature (1800.degree. F. to 2800.degree. F.) injection of dry, pulverized limestone has also been used to reduce sulfur emissions. In addition, several recant investigations have shown that hydrated lime (Ca(OH).sub.2 ) is effective in reducing SO.sub.x emissions. None of the current literature, however, shows that dry sorbent injection can be directly combined with reducing agent injection to achieve optimum NO.sub.x and SO.sub.x control simultaneously and with relatively small capital cost. Such a process would be a major advancement in the art.
From the discussion above, it is apparent that what is currently needed in the art are methods for the selective, noncatalytic reduction of NO.sub.x which produce NO.sub.x emissions well below those obtainable using prior art methods. It would be an advancement in the art to provide such methods which employed a process which effectively produced emission levels below 100 ppm without a catalyst and using inexpensive and readily available reactants. It would be a further advancement in the art to provide methods for simultaneously controlling NO.sub.x and SO.sub.x emissions.
Such methods are disclosed and claimed below.